This invention relates to modified supported chromium oxide olefin polymerization catalysts.
It further relates to a method of making ethylene polymers and copolymers.
Supported chromium oxide catalysts have been used for many years in the polymerization of olefins. Ethylene can be polymerized by contacting the monomer with a silica-supported chromium oxide catalyst, the reaction being carried out in an inert liquid at temperatures below 130.degree. C. for producing solid polymer suspended in the liquid or at temperatures above 130.degree. C. for solution polymerization. The properties of the resulting polymer depend upon a number of factors, including the type of catalyst employed and its activation temperature, the reaction pressure, and the reaction temperature. It is generally known that titanium can be added to the supported chromium oxide catalyst to produce a polymer having an increased melt index. It is also generally known that certain substances called promoters or adjuvants can be used in combination with chromium oxide catalysts to modify the properties of the polymer. The use of chromium catalysts with certain organoboron promoters is known generally to broaden the molecular weight distribution and improve the environmental stress crack resistance of polymers made using these catalysts as well as to increase the catalyst productivity.
Attempts have been made to obtain ethylene polymers having the high productivity and the improved environmental stress crack resistance imparted by organoboron promoters and the increased melt index potential obtained with chromium oxide-silica catalysts containing titanium. When commercial cogel catalysts containing 2.0 to 2.5 weight percent titanium were employed with triethylborane (TEB) as a promoter, the resulting polymers exhibited reduced density, which resulted in ethylene polymers which lacked the high stiffness desired for blow molding and injection molding applications. It is believed that the use of the organoboron compound with chromium oxide catalysts containing about 2.5 weight percent or less titanium results in the production of a small amount of 1-butene and 1-hexene from the ethylene monomer. These higher olefins are incorporated into the polymer chain, reducing the density by disrupting the linear polymer structure. In polymerization processes in which ethylene monomer is recycled to the reactor, the presence of these higher olefins may necessitate a fractionation step to separate the accumulated butene and hexene from the ethylene monomer. When ethylene is copolymerized with other monomers, the generation of higher olefins in the reactor complicates the process of maintaining the ethylenecomonomer ratio and thereby producing polymer having a predictable density.
It is therefore an object of this invention to provide an improved chromium oxide polymerization catalyst.
It is a further object to provide a process by which high-density ethylene polymers having a high melt index and good stress crack resistance are prepared in high yield.
It is a further object of the invention to minimize the production of higher olefins during the polymerization process.